1. Field of the Invention
The present invention generally relates to an optical disk apparatus for recording and/or reproducing information to and/or from an optical disk, and in particular to an optical disk apparatus having an aberration or tilt correction control system for maintaining an orthogonal relationship between an optical axis of a light beam projected by an optical pickup device and an information recording surface of the optical disk.
2. Description of the Prior Art
High speed random access is possible with disc-shaped recording medium, and a high recording density can be achieved by formatting a disk with a narrow data track pitch and pit pitch.
Optical disks such as a DVD, which is typical, have been widely used in recent years as a large capacity recording medium because of a high recording density. Further advances in recording density have also been achieved to further increase storage capacity. Optical disks, however, are typically made from low rigidity materials such as polycarbonate, and even disk deflection or deformation due to warp or bending caused by the dead weight of the disk per se cannot be ignored.
In the case where original data recorded on, for example, an optical disk medium is reproduced by means of an optical pickup device, if an optical axis of an object lens of the pickup device is not perpendicular to the surface of the disk but inclined at some angle with respect to the orthogonal relationship to the surface of the disk, the beam spot projected onto the disk surface is distorted due to the aberration thereof, and there may undesirably cause a distortion in a waveform of the reproduced signal output of the optical pickup device. It is noted here that the inclination error with respect to the orthogonal relationship is referred to as “orthogonal shift” or “tilt”, hereinafter.
Moreover, due to the aberration distortion caused in the beam spot, the reflection light beams reflected from adjacent recording pits formed on the disk undesirably interfere with each other. Thus, the modulation degree of the reproduction signal is deteriorated, and there may be caused a shift error in a peak time (referred to as “peak shift” hereinafter) of the reproduction signal, resulting in occurrence of a discrimination error of the information reproduction (RF) signal to be a problem.
As a method of removing distortion components included in the reproduction signal waveform, there has been conventionally used an adaptive equalizer utilizing a finite impulse response filter (referred to as “FIR filter”, hereinafter). In this method, an adaptive equalization is recently carried out in a digital data processing system by previously quantizing the reproduced signal using an A/D converter.
Particularly in recent years, however, the data recording density on the recording medium has been remarkably increased and distortion of the reproduction signal due to inter-code interference of the recorded data on the medium is increased, and also a noise influence in a data transmission path can not be ignored because of reduction in amplitude of the reproduction signal. Also, the reproduction signal is sensibly deteriorated by a slight tilt of the disk or a defocus condition in the playback system.
Specifically, in the case of using an optical disk such as DVD-RAM for compatibly recording and reproducing information, such a tilt and defocus condition badly affect both the recording and reproducing operations. Therefore, the aberration correction must be performed with a further higher accuracy. In this situation, as a method of removing the aberration distortion components included in the reproduction signal waveform, a tilt control or aberration correction control method is effective for maintaining an orthogonal relationship between the optical disk surface and the optical axis of the optical pickup device.
A conventional tilt control apparatus is suggested, for example, in the Japanese Laid-Open Patent Publication No. Tokkai-Sho 61-51630, which discloses that the tilt control apparatus detects an error in the orthogonal relationship between a disk surface and an optical axis of a light beam irradiated onto the disk by using tilt photo-sensors, thereby maintaining the orthogonal relationship based on the orthogonal error signal detected by the photo-sensors.
However, this conventional tilt control apparatus has following problems. Specifically, since a pair of photo-sensors are disposed on the right and left sides of the object lens of the optical pickup device, and the apparatus is made large in size and complicated in construction. Moreover, if the characteristics of the photo-sensors are varied in time lapse, an offset is generated to cause an error in the orthogonal relationship. Therefore, a normal orthogonal relationship cannot be maintained even if the orthogonal error is set to zero. Also, a tilt control cannot be performed in a circumferential direction (i.e., track tangential direction) of the disk.
In order to solve these problems, another conventional tilt control system is suggested, for example, in the Japanese Laid-Open Patent Publication No. Tokkai-Hei 5-174406, which discloses that the orthogonal shit error in the circumferential (track tangential) direction included in the reproduction signal is detected and corrected based on the shift in the peak time (i.e., peak shift) of the reproduction signal during the operation of reproducing the information from the disk in a pulse phase modulation (PPM) system.
However, in recent optical disk apparatuses, the information is mainly recorded on the disk in a pulse width modulation (PWM) system in order to obtain a higher recording density. Therefore, there has been a problem that the above conventional tilt control system adapted to the PPM recording system can not be utilized in the data reproduction system adapted to the PWM recording system. Moreover, there has been a problem that a tilt correction control in the circumferential (.e., track tangential) direction of the disk can not be performed during the data recording operation.